1. Field of the Invention
This invention relates to a parallel multichannel optical communications module which transmits a plurality of signals via an optical fiber having a plurality of element fibers. The optical communications module is a concept including a laser diode module, a photodiode module, a modulator module, a demodulator module or an in-line monitor.
This invention claims the priority of Japanese Patent Application No. 2002-100931 filed on Apr. 3, 2002, which is incorporated herein by reference.
2. Description of Related Art
Planar lightguide type (PLC; planar lightguide circuit) modules having a plurality of lightpaths and optoelectronic device chips (LDs or PDs) are promising modules for coping with forthcoming multichannel optical communications which bring four signals, eight signals or sixteen signals simultaneously via four channels, eight channels or sixteen channels. The PLC modules would be low-cost, small-sized optical communications devices.
Access of a plurality of laser diodes (LDs) which consume currents of tens of milliamperes induces a variety of drawbacks of inter-channel interference, crosstalk and degradation caused by concentrated heating. Multicore ribbonfibers with a plurality of element fibers have a standardized pitch of 250 μm. The pitch is defined to be a distance between a center of a fiber and a center of a neighboring fiber in a ribbonfiber. A problem of a multichannel communications is a narrow pitch of 250 μm of multichannel fibers. A laser diode (LD) or a photodiode (PD) has a size from 300 μm to 500 μm at present. The narrow pitch prohibits a PLC module from aligning laser diodes in parallel at ends of the 250 μm pitch multichannel lightpaths. A contrivance was proposed for enlarging the pitch by curving lightpaths outward.
{circle around (1)} M. Shishikura, K. Nagatsuma, T. Ido, M. Tokuda, K. Nakahara, E. Nomoto, T. Sudoh and H. Sano, “10 Gbps×4-channel parallel LD module”, Proceeding of the 2001 Electronics Society Conference of IEICE, C-3-50, p 160
FIG. 33 shows a four channel LD module with enlarging lightwaveguides proposed by {circle around (1)}. This is an example of four channel transmitting modules (LD modules) which can be employed in multiwavelength communications networks. The LD module is built on a bench 70. The bench 70 has a lightwaveguide layer 72 on a top. Four lightwaveguides 73, 74, 75 and 76 are produced on the lightwaveguide layer 72. Initial ends of the lightwaveguides have a pitch of 250 μm which is equal to the standardized pitch (H=250 μm) of ribbonfibers. The pitch of the lightwaveguides increases in an intermediate portion along a longitudinal direction. Final ends have a wide pitch of 1000 μm. Laser diode (LD) chips 77, 78, 79 and 80 are furnished at a 1000 μm pitch on extensions of the final ends of the lightwaveguides. The wide 1000 μm pitch secures wide spatial separation between neighboring laser diodes. {circle around (1)} asserted that crosstalk between neighboring channels (channels 1-2, channels 2-3 and channels 3-4) is less than −40 dB at a frequency of 10 GHz.
The known reference {circle around (1)} solved a problem of the narrow pitch of ribbonfibers by enlarging a width between neighboring channels to a 1 mm pitch which enables the LD module to mount laser diodes chips on extensions of the final ends of the lightwaveguides. {circle around (1)} succeeded in reducing crosstalk between the neighboring laser diodes by the enlarged separation.
Exploitation of PLC structures has been desired for reducing cost, increasing productivity and enhancing performance of LD modules, PD modules and LD/PD modules. Low cost, small-size, high-performance and large-scale production is indispensable for prevalence of optical communications subscriber networks. Nobody has yet suggested multichannel LD/PD modules built upon the PLC technique. There are, however, some proposals of single-channel LD/PD modules based upon lightwaveguides.
{circle around (2)} Japanese Patent Laying Open No. 11-68705, “Two-way WDM optical transmission reception module”
proposed a single-channel LD/PD module of a PLC type having a y-branched light waveguide made upon a silicon bench. The y-branched waveguide occupies a wide area on a silicon bench. Due to the space-consuming y-branch a structure of the module proposed by {circle around (2)} cannot be extended to multichannel LD/PD modules for multichannel communications.
Pervasion of optical subscriber networks requires low cost and size-reduction of station LD/PD modules in addition to low cost, small-sized subscriber modules (ONU; optical network unit) which include only a single channel with a single laser diode and a single photodiode.
Many subscribers are connected to a single central station by fibers. The number of subscribers is denoted by M. M fibers are required for connecting M ONUs (subscribers) to a station. The station should have M pairs of laser diodes and photodiodes for exchanging 2M signals with M ONUs. Transmission of a plurality of signals requires ribbonfibers (tapefibers) which have 4, 8, 16 or 32 parallel element fibers in a tape.
The central station requires multichannel optical communications modules for matching with the ribbonfibers (tapefibers). If the station relied upon single-channel modules, M single-channel modules should be installed at the station for exchanging bi-directional signals with M ONUs, which would occupy very wide space in the station. If the station uses m-channel modules, the number of station modules is reduced from M to M/m. Reduction of the module number would curtail space for installing modules in the station. Multichannel modules are essential for the central station.
Most prevalently used ribbonfibers have a pitch of 250 μm. Four, eight, twelve or sixteen channel ribbonfibers have all the standardized 250 μm pitch. A plurality of element fibers align in a plane at a common pitch of 250 μm in popular flat ribbonfibers.
Current ribbonfibers (=tapefibers) have a determined, common pitch of 250 μm. The 250 μm pitch is too narrow to make multichannel LD or PD modules by aligning laser diodes or photodiodes just along extensions of the element fibers. The pitch should be enlarged, for example, to 500 μm to 1000 μm for securing enough space for laying LDs or PDs. Wide separation with margins will solve difficulties of crosstalk or thermal diffusion.
Current laser diodes or photodiodes have a width more than 250 μm in a lateral direction vertical to an axial line. Optoelectronic devices (LDs & PDs) have, in general, a lateral width from 300 μm to 500 μm for the sake of facile production, feasible mounting, easy handling and sufficient thermal diffusion.
Ribbonfibers (tapefibers) have a pitch H=250 μm too narrow for mounting devices. Installation of laser diodes or photodiodes requires pitch enlarging portions in lightpaths on a bench for coupling to a channel number of the laser diodes or photodiodes. The curving pitch enlarging portions of lightpaths will require a long and wide silicon bench having a length of 10 mm to 15 mm and a width of 6 mm to 15 mm, which depends upon the channel number. Long and wide silicon benches will cause high cost, large sized multichannel modules.
The narrow width of a 250 μm pitch of standardized ribbon fibers restricts design of optoelectronic device chips or lightwaveguides. Widening of the 250 μm pitch of ribbonfibers by giving strong curvature to lightwaveguides on silicon benches causes difficulty on designing and producing of optoelectronic (LD or PD) modules, which induces a bad yield of producing modules.
Sometimes lightwaveguides formed on a bench are not terminated by photodiodes or laser diodes. An intermediary photoactive device, which is neither an LD module nor a PD module, provides a bench with wide pitch lightwaveguides or fibers overall furnished on the bench and installs photoactive devices halfway on the lightwaveguides for giving some processing to signal beams propagating in the lightwaveguides and allowing the processed beams to go out of the lightwaveguides. In the case, a wide pitch of 500 μm to 1000 μm, on the intermediate device should be restored down to the original 250 μm pitch again. The intermediate module would require twice changes of pitches by making width enlarging curving lightwaveguides and width decreasing curving lightwaveguides. Change of the pitch increases difficulty of fabrication, raises cost of manufacturing, and reduces yield of products.
Prevalent multichannel ribbonfibers have a narrow, standardized pitch of 250 μm. Four, eight, twelve or sixteen element fibers are aligned at a 250 μm spatial period in a flat ribbonfiber.
It is difficult to mount laser diodes or photodiodes at points aligning at a pith smaller than 250 μm, because current laser diodes and photodiodes have a side larger than 300 μm at present. Discrepancy between the chip size and the channel pitch forces multichannel modules to bend lightwaveguides on a bench for securing wide space for chips. If a multichannel module is made without bending lightpaths on a bench, the multichannel module with only linear lightpaths will be a quite excellent contrivance for the future of optical communications.
A purpose of the present invention is to provide a linear multichannel surface mountable type module without curving lightpaths. Another purpose of the present invention is to provide a linear multichannel surface mountable type module having linear lightpaths which can join with a multichannel connector having a ribbonfiber of the conventional 250 μm pitch. A further purpose of the present invention is to provide a linear multichannel surface mountable type module having linear lightpaths on which photoactive devices other than laser diodes or photodiodes can be furnished.
This invention includes two types A and B. Type A means a module in which lightpaths are terminated by optoelectronic device chips. Type B means a module in which lightpaths are overall formed on a bench and are not terminated by optoelectronic devices.
[Type A (Lightpaths Terminated by Optoelectronic Devices)]
The present invention proposes a type A module having a bench, m lightpaths having a width d, having one of n kinds of lengths which is different from the lengths of neighboring lightpaths and aligning with a pitch E equal to the fiber pitch H formed on the bench, and m optoelectronic device chips having a width W which satisfies an inequality of E<W<2E−d and being placed behind final ends of the lightpaths on the bench. Type A ensures sufficient space for mounting chips with enough margins by making different lengths of lightwaveguides and preparing longitudinally different sites for device chips.
The lightpath means a lightwaveguide or an optical fiber formed on a bench. The optoelectronic device means a laser diode (LD) or a photodiode (PD). A laser diode is sometimes written as a laser or an LD in short. A photodiode is sometimes abbreviated to a PD here. This invention includes a laser diode (transmitting) module and a photodiode (PD; receiving) module according to the kind of the optoelectronic devices (laser or photodiode). This invention differentiates longitudinal positions of neighboring optoelectronic devices on neighboring lightpaths. The sites of neighboring optoelectronic devices are different in a longitudinal direction. Discrepancy of the longitudinal positions of the optoelectronic device sites ensures about twice of the channel pitch for mounting the device chips. What is forbidden is the same longitudinal sites for a pair of the most neighboring devices. The next-neighboring sites can take the same longitudinal positions. There are n different longitudinal positions and m lightpaths on a bench. The number of allowable sets of chip sites is n(n−1)m−1.
[Type B (Lightpaths Not Terminated by Optoelectronic Devices)]
The present invention proposes a type B module having a bench, m lightpaths having a width d, aligning with a pitch E equal to the fiber pitch H formed overall on the bench, m optoelectronic device chips having a width W which satisfies an inequality of E<W<2E−d and being placed midway at sites which are different from neighboring sites in a longitudinal direction on the lightpaths on the bench and optionally m photoactive devices installed upon the lightpaths. Type B ensures sufficient space for mounting chips with enough margins by preparing longitudinally different sites for neighboring device chips.
Photodiodes have three different versions, that is, a top incidence type, a bottom incidence type and a front incidence type. Front incidence type photodiodes should be epi-down mounted on sites formed upon a bench by leveling an emission stripe with cores of the lightpaths. In the case of bottom incidence type photodiodes, wavelength selective filters should be inserted into the lightpaths and the photodiodes should be mounted above lightpaths just before the wavelength selective filters in order to guide the beams reflected by the wavelength selective filters into bottoms of the photodiodes.
In the type B, there are m lightpaths and n different longitudinal positions for sites of devices. Any site should be different from neighboring sites in the longitudinal direction. The number of allowable sets of sites is n(n−1)m−1.
Optionally furnished photoactive devices are polarizers, isolators, wavelength selective filters, monitoring photodiodes, rod lenses, gratings, photomodulators, photoamplifiers and so forth.